LV generator protection - engineering site - Schneider Electric
LV generator protection - engineering site - Schneider Electric
LV generator protection - engineering site - Schneider Electric
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Protection and Monitoringof a <strong>LV</strong> Generator Set3.1.2.2. Calculating the short-circuit currentManufacturers normally specify the impedance values and time constantsrequired for analysis of operation in transient or steady state conditions.Impedance table: Leroy Somer <strong>generator</strong>(kVA) 75 200 400 800 1600 2500x"d (%) 10.5 10.4 12.9 10.5 18.8 19.1x'd (%) 21 15.6 19.4 18 33.8 30.2x'd (%) 280 291 358 280 404 292Resistances are always negligible compared with reactances.The parameters for the short-circuit current study are:Value of the short-circuit current at <strong>generator</strong> terminalsShort-circuit current strength in transient conditions is:sorsU Nis the <strong>generator</strong> output phase-to-phase voltage (Main source).Note: this value can be compared with the short-circuit current at the terminalsof a transformer. Thus, for the same power, currents in event of a short-circuitclose to a <strong>generator</strong> will be 5 to 6 times weaker than those that may occur with atransformer (main source).This difference is accentuated further still by the fact that <strong>generator</strong> set power isnormally less than that of the transformer.ExampleE79474EMVGS<strong>LV</strong>NCNCMain/standbyNon-priority circuitsPriority circuitsNC: normally closed.NO: normally open.Figure 15.When the <strong>LV</strong> network is supplied by the Main source 1 of 2000 kA, the shortcircuitcurrent is 42 kA at the main <strong>LV</strong> board busbar. When the <strong>LV</strong> network issupplied by the Replacement Source 2 of 500 kVA with transient reactance of30 %, the short-circuit current is made at approx. 2.5 kA, i.e. at a value 16 timesweaker than with the Main source.18